There are definitely themes in ATLA based on Indian cultures, such as the name of the show itself, but most of the characters are more based on East and Southeast Asian cultures. Dunno why things ended up that way. Lots of the characters are lighter skinned like the actors but Katara and Sokka are definitely exceptions. Both actors are indigenous North American but their animated versions were much darker. As a fan of the show, I think the actors seem like good choices so far but I can see it being seen as lightwashing for sure. Having more South Asian looking characters would feel very normal in-universe so they could do that in later seasons.